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The thermal and structural evolution of the Ronda peridotite during exhumation

THE THERMAL AND STRUCTURAL EVOLUTION OF THE RONDA PERIDOTITE
DURING EXHUMATION
by
Katharine Elizabeth Johanesen
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(GEOLOGICAL SCIENCES)
December 2011
Copyright 2011 Katharine Elizabeth Johanesen

The Ronda peridotite massif of southern Spain exposes subcontinental lithospheric mantle that records pressure-temperature data and microstructures formed during exhumation beneath the rapidly extending Alboran domain. The peridotite is zoned from garnet- and spinel-bearing mylonites at the structural top, to spinel-bearing tectonites, to melt-percolated spinel-bearing granular peridotites, to plagioclase-bearing tectonites at the structural base. I find microstructural evidence of melt present in the spinel zones prior to the deformation event which exhumed the peridotites, and I, therefore, reinterpret the spinel tectonites as being a result of deformational overprinting of part of the granular domain. I also reinterpret garnet intergrown with spinel in the mylonite zone as part of the pre-mylonitic porphyroclast assemblage, rather than as a syn-mylonite assemblage. This places mylonite formation within the spinel field, rather than right on the garnet-spinel transition (18 kb). Two-dimensional thermal modeling indicates that these conditions require removal of lithospheric mantle below 100 km followed by exhumation along a low angle shear zone. ❧ Olivine crystallographic lattice preferred orientations (LPO) from these samples fall into three categories: A-type with a maximum of (010) poles normal to foliation and bands of (100) and (001) parallel to foliation, B-type with (010) poles normal to foliation and (100) parallel foliation and normal to lineation, and E-type with (100) parallel to lineation and (010) parallel foliation and normal to lineation. The garnet- and spinelbearing mylonites and recrystallized grains in the spinel tectonites show A-type LPO, indicating dry conditions and moderate stresses during deformation. Coarse grains in the spinel tectonites and granular peridotites have a B-type LPO indicative of higher stresses and possible water contents. A shear zone that cross-cuts the granular peridotites has an E-type LPO, which suggests high water contents but similar stress conditions of the A-type fabrics. Neoblasts in the mylonite and tectonite zones both have an average grain size of 110 μm. Recrystallized grain size piezometry yields 40 MPa deviatoric stress during. ❧ The path of cooling and deformation of the mylonite and deeper units can be explained by removal of lithosphere below 100 km, followed by rapid exhumation on the order of 10-20 km/my along an initially 4 km wide shear zone (encompassing the mylonites and tectonites) dipping at 30°. The earliest stages of this shear zone are likely related to the horizon of lithospheric removal. It is possible that lithospheric mantle removal was localized into a zone of weakness, which was then exploited by as a zone of localization for a lithosphere-scale shear zone. As it cooled to 800-850 °C during exhumation, the shear zone was localized to ~500 m width, producing the mylonites and likely increased in slip rate during this process.

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THE THERMAL AND STRUCTURAL EVOLUTION OF THE RONDA PERIDOTITE
DURING EXHUMATION
by
Katharine Elizabeth Johanesen
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(GEOLOGICAL SCIENCES)
December 2011
Copyright 2011 Katharine Elizabeth Johanesen